Endosymbionts and their hosts have gone very far in mutual integration. Endosymbionts live directly inside the bodies of their hosts. In the vast majority of cases, the symbiosis of the two organisms is so close that one species cannot survive without the other one and does not even occur in isolated form in nature – with the exception of the invasive stage of an endosymbiont and new-born young of the host, which have not had time to become “infected” by their parents or the environment. Ruminant ciliates and bacteria are known examples of endosymbionts, as are endiosymbiotic protozoa in the digestive tracts of termites. In the absence of these organisms, the host organism would not be capable of utilizing its main source of food, plant polysaccharides, especially cellulose. However, symbionts in the digestive tract are also important for other species, such as humans. Experiments with rodents freed of microbial symbionts and kept permanently under these conditions have shown that these animals require, for their lives, approximately one third more food than animals kept under normal conditions (Hooper & Gordon 2001). The symbiosis of fungi and algae (or blue-green algae) in the form of lichens is an obvious textbook example. It is less widely known that, according to some (very bold) ideas, terrestrial plants are also actually a sort of inverse lichens, i.e. the products of ancient symbiosis between algae of the Charophyceae genus and a fungus. Here, the algae would provide the mechanical support, protection against UV radiation and a number of other functions and the fungus would provide the cytoskeletal apparatus required to prolong the growth of cells, employed, for example, in the growth of pollen tubes and hair roots (Atsatt 1993; Jorgensen 1993)
Thus, very few species of animals can survive in the absence of endosymbiotic organisms. If nothing else, the symbionts at least provide them with some essential substances, for example some vitamins, that they are not capable of synthesizing themselves. However, in a great many cases, cooperation amongst the relevant species of organisms need not be especially close; the individual species of endosymbionts and the individual species of hosts can be mutually substituted.